Not All Amino Acids Are Subject to Hepatic Destruction
During the absorptive period, amino acids for peripheral (non- hepatic) protein synthesis must come from that portion of amino acids that escape hepatic destruction. As seen from Figure 32-7, this portion amounts to only about 23% of the amino acids absorbed from the gut.
Although this may seem like a meager portion of amino acids to be allocated for protein synthesis by all body tissues except liver, two considerations make it more appropriate. First, amino acids are selectively taken up by the liver, so the distribution of individual amino acids in blood leaving the liver is not the same as that in blood reaching the liver. The indispensable amino acids, especially the BCAAs, are not avidly extracted by the liver, whereas some of the dispensable amino acids (e.g., alanine) are extensively taken up by hepatic tissue. The dispensable amino acids can be synthesized by proteinproducing tissues; thus the relatively low concentration ot serum amino acids resulting from hepatic amino acid removal is not rate limiting for tissue protein synthesis. Second, the proportion of amino acids taken up by the liver, as well as the fate of the amino acids that are taken up, is not constant and can be adjusted according to the body's protein needs. Low- protein diets lead to reductions in hepatic amino acid uptake, protein synthesis, and amino acid destruction by the liver.
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